Metals that form alloys or intermetallic compounds with lithium have been considered for use as alternatives to lithium metal or carbon electrodes in lithium electrochemical devices such as lithium ion batteries. Examples of such metals include Al, B, Bi, Cd, In, Ga, Pb, Sb, Si, Sn, Zn, or mixtures of these metals, as reviewed by R. A. Huggins (J. Power Sources, 26 109-120 (1989) and D. Fateaux and R. Koksbang (J. Appl. Electrochemistry, 23, p. 1 (1993)).
Even those metals that do alloy with lithium to high concentrations, for example reaching a lithium to metal molar ratio of at least one, have severe limitations as an electrode material, as discussed in the article by D. Fateaux and R. Koksbang. A major limitation is the fact that the formation of a highly lithiated alloy or compound from a metal is accompanied by a large volume expansion. The volume expansion of the alloy or compound relative to the molar volume of the starting metal increases greatly with lithium concentration, such that the compositions that are most desirable due to having the highest lithium storage have the largest expansion. Amongst numerous metals that can alloy with lithium to a high concentration, the lithiated alloy can easily expand to a molar volume that is 1.5 to 5 times that of the starting metal. It is widely recognized by those skilled in the art that this volume expansion, and the subsequent volume contraction which occurs when lithium is electrochemically removed, is undesirable as it often causes mechanical fracture of the metal electrode, resulting in decreased lithium storage capability. The lithium-concentration-dependent volume change is particularly detrimental in electrochemical devices that undergo cycling, in which lithium is repeatedly removed from and inserted into the metal electrode. A rechargeable lithium battery is one such device.
Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, (Science, 276 1395 (1997)), and T. Kubota and M. Tanaka (U.S. Pat. No. 5,654,114) have described lithium battery anodes based on metal oxides, primarily tin oxide, that are capable of reversible charge capacity of approximately 600 mAh/g (2200 mAh/cm3). The primary drawback of this type of material has been the high first-cycle irreversible capacity loss, typically 400-600 mAh/g, due to the consumption of lithium in oxide-forming reactions during the first lithium insertion. These anodes are prepared as an oxide, not as a metal or a metal-oxide composite, and do not demonstrate the advantageous features which result from the processes of partial oxidation or reduction as described herein below prior to being assembled as a battery.
O Mao et al., (Electrochem. Sol. St. Lett., 2[2]3-5 (1999)) have described Sn/Fe/C composites of several nanometer individual phase particle size produced by mechanical alloying. The composites demonstrate a lower first-cycle irreversible capacity loss and also lower reversible capacity on a weight and volume basis (200 mAh/g, 1700 mAh/cm3) than the tin oxides. These electrode materials have an inactive phase of high density, detracting from their capacity, and rely upon fine size alone for their improved properties relative to bulk metals. They do not utilize the beneficial methods of partial reduction or oxidation in their preparation.
Thus, the electroactive metal oxides and metals of the prior art do not provide a mechanically robust material having the desired reversible charge capacity for use in cyclically operated electrochemical devices. New materials and methods of their manufacture are needed.
One objective of the invention is to provide an electroactive material having a robust microstructure that can accommodate the large volume changes associated with electrochemical insertion or removal of lithium.
Another object of the invention is to provide an electrode and other electrochemical devices such as rechargeable batteries which contain a mechanically robust electroactive material and which are resistant to mechanical failure of the electrode.
It is also an object of the invention to provide an electroactive material having a high reversible charge capacity for us in cyclically operated electrochemical devices such as a rechargeable battery.
Still another object of the invention is to provide a method of making an electroactive material which is sufficiently mechanically robust to withstand the internal stresses experienced during the large volume changes which occur during electrochemical insertion or removal of lithium.
These and other objectives are realized in the materials and methods of preparation comprising the invention, as is substantially described herein.
In one aspect of the invention, a composite material is provided having a first material that is an elemental metal, metal alloy, metal oxide, or other metal compound, selected so that it is able to alloy with lithium or other element such as hydrogen, potassium, sodium and the like, and prepared in a dispersed one-, two- or three-dimensional form. The first material is intimately mixed with or dispersed within a second material that provides mechanical support for the first material and, optionally, is substantially conductive to electrons or electron holes or lithium ions. The composite material may possess void spaces, or it may experience internal stress. Either feature may be advantageous in increasing the reversible charge capacity of the material.
By the term xe2x80x9cdispersed,xe2x80x9d it is understood that the smallest dimension of the metal or metal compound that is less than 10 micrometers, preferably less than 5 micrometers, and more preferably still less than 1 micrometer. The dispersed material is substantially surrounded by the supportive second material.
By the term xe2x80x9celectroactive,xe2x80x9d it is understood to mean that during operation of the device the metal, alloy, or compound stores electrical charge by forming an alloy with lithium or other element such as hydrogen, potassium, sodium and the like.
In a preferred embodiment of the invention, the composite material includes a first material that is an elemental metal or metal alloy, selected so that it is able to alloy with lithium or other element such as potassium, sodium and the like, and prepared in a dispersed one-, two- or three-dimensional form. The first material is intimately mixed with or dispersed within a metal oxide that provides mechanical support for the first material and, optionally, is substantially conductive to electrons or electron holes or lithium ions. The metal oxide may be glassy or crystalline. Crystalline metal oxide systems are currently preferred since they simplify the selection process, their ionic and electronic transport properties being generally known. However, glassy oxides can have equally advantageous transport properties. In preferred embodiments, the crystalline metal oxide is a normal spinel, inverse spinel, or disordered spinel structure compound MeIIcXd where 0.5 less than c/d less than 1, or a rutile structure compound MeIIxOy where 0.5 less than x/y less than 1 or an ordered or disordered derivative of this structure type, or a corundum or ilmenite structure compound MeIIxOy where 0.5 less than x/y less than 1 or an ordered or disordered derivative of this structure type, or a perovskite structure compound MeIIxOy where 0.5 less than x/y less than 1 or an ordered or disordered derivative of this structure type.
In other preferred embodiments, the crystal structure is selected to promote ion, e.g., lithium ion, transport and electronic conductance.
In preferred embodiments, the composite may be prepared by the process hereafter referred to as xe2x80x9cpartial reduction,xe2x80x9d xe2x80x9cpreferential reduction,xe2x80x9d or xe2x80x9cinternal reduction,xe2x80x9d or the process hereafter known as xe2x80x9cpartial oxidation.xe2x80x9d These methods of preparation confer numerous benefits as described herein. Partial reduction or oxidation refers to processes in which only a portion of the material is reduced or oxidized, respectively. By internal reduction, the term is understood to mean a process in which the reduction product, i.e., a metal, forms as a discrete phase within or internal to the starting material phase, which may be a single crystalline or multicrystalline material. By partial reduction, the term is understood to include internal reduction, but also to include processes in which the reduction product, i.e., a metal, nucleates or forms as a discrete phase or precipitate at surfaces, such as the surface of a powder particle. Both processing methods are observed to provide the materials and advantages of the invention.
In one aspect of the invention, a composite material for use as an energy-storage material is provided which includes a first material comprising one or more metals, metal alloys or metal compounds containing MeI capable of alloying with a species selected from the group consisting of alkali metals and hydrogen, the first material intimately mixed with a matrix of a second material comprising a metal compound MeIIX, produced by partial reduction of a mixed-metal composition MeIaMeII1xe2x88x92aXz, where 0.1 less than a less than 0.9, z greater than 0, and X is one of oxygen, boron, carbon, nitrogen, phosphorus, fluorine, chlorine, bromine, or iodine, under conditions that preferentially reduce MeI over MeII.
In another aspect of the invention, a composite material for use as an energy-storage material includes a first material comprising one or more metals, metal alloys or metal compounds containing MeI capable of alloying with a species selected from the group consisting of alkali metals and hydrogen, the first material intimately mixed with a matrix of a second material comprising a metal compound MeIIX, produced by partial oxidation of a mixed-metal composition MeIaMeII1xe2x88x92aXz, where 0.1 less than a less than 0.9, z greater than 0, and X is one of oxygen, boron, carbon, nitrogen, phosphorus, fluorine, chlorine, bromine, or iodine, or a starting mixed-metal alloy MeIaMeII1xe2x88x92a, where 0.1 less than a less than 0.9 under conditions that preferentially oxidize MeII over MeI.
A composite material is prepared by subjecting a starting mixed-metal compound to a chemical or thermochemical treatment to convert a component thereof into a first material, which is xe2x80x9clithium-activexe2x80x9d wherein the term is understood to mean that it alloys with lithium during operation of the electrochemical device. The thermochemical treatment causes the volume occupied by the first material to decrease relative to the volume it formerly occupied (as the precursor component) and relative to a second material, which is also a component of the mixed-metal compound and non-reactive, or only partially reactive, under the conditions of the thermochemical treatment. The second material provides a rigid mechanical frame work, and/or provides electronic conductivity, and/or provides lithium ion conductivity to the first material. The second material can also store lithium although it is not required to do so.
Taking for example a metal compound that decreases its molar volume upon being reduced, according to the invention an electroactive material is prepared by chemical or thermochemically preferentially reducing certain components of a starting mixed-metal compound in order to produce a first material of decreased molar volume, or by preferentially oxidizing certain components of the starting mixed-metal compound to produce a second material of increased molar volume (both relative to the volume initially occupied by the respective precursor metal compound in the mixed-metal starting material.) Some metal compounds expand in volume upon reduction, in which case the choice of reduction or oxidation is selected so as to achieve the desired volume change depending on whether the metal comprises the first material or second material. Herein, molar volume is understood to be the volume of an amount of the metal, metal alloy, or metal compound containing one mole of metal atoms. In the instance of a metal alloy or mixed-metal compound, the mole of metal atoms is understood to contain each of metals in the same proportion as is present in the alloy or compound.
In another aspect of the invention, electrodes are provided. The composite material may be used as an electrode in lithium ion electrochemical devices such as lithium batteries and electrochromic windows, mirrors, or displays, or electrochemical devices that use the material as an electrode or storage material.
Thus, one embodiment of the invention includes using a starting mixed-metal composition MeIaMeII1xe2x88x92aXz, where 0.1 less than a less than 0.9, z greater than 0, and X is one of oxygen, boron, carbon, nitrogen, phosphorus, fluorine, chlorine, bromine, or iodine to obtain an electroactive composite material. The starting composition may be a single-phase material, i.e. a solid solution, or a mixed-phase material, i.e., a microphase separated material or a combination of individual phases as is discussed hereinbelow. It is chemically or thermodynamically, partially or internally reduced so as to produce a fine metal dispersion enriched in MeI, i.e., the first material, intimately mixed with or contained within a metal compound more enriched in MeII and X than the first material, i.e., the second material. The term reduction is understood to mean a decrease in the value of z, or a decrease in the oxidation state of any one of the metals. The metals MeI include one or more metals that have a less negative Gibbs free energy for the formation of an alloy or compound with an element X than the metals that comprise MeII do with X. Alternatively stated, the metals MeI are preferentially reduced relative to the MeII metals. The second material may also subsequently alloy with lithium, although according to the invention it is not necessary that it do so.
In preferred embodiments, X is oxygen and MeI is one or more of Ag, Sb, Sn, Cu, In, Ge, Zn, Ga, B, and Si, and preferably is Ag, Sb, Zn or Sn. In other preferred embodiments, MeII is one or more of Cu, Mn, Sb, Ni, Co, Fe, In, Ge, Zn, Ga, Cr, V, B, Si, Ti, Ta, Nb, Ru, Ce, Al or Mg, and preferably is V, Ti, Mn, Ni, Co or Fe. MeII is further selected to have a more negative. Gibbs free energy of metal oxide formation than MeI. When the starting compound is a mixed-metal oxide, the metal or metals having the least negative Gibbs free energy of formation amongst the metal oxides comprising the starting mixed-metal oxide will be preferentially reduced to the zero-valent state to form the dispersed metal phase or phases of the composite material of the invention. The oxide comprising the second material is preferably a transition metal oxide, or a mixed-metal oxide in which the metal fraction is at least 10% by mole a transition metal, preferably at least 25%, 50%, 75% or at least 90% molar composition. After the partial reduction process, the material is useful as a lithium-active component of an electrode in a lithium electrochemical device.
In a preferred embodiment, the mixed-metal compound comprises MexIMn3xe2x88x92xxe2x88x92XO4xe2x88x92y spinel, x being between about 0.5 and 2 and y being between zero and about 0.5, and the metal MeI after reduction is Ag, Sb, Zn, or Sn, and manganese alloys thereof.
In another preferred embodiment, the starting mixed-metal composition has formula MeIbMeII2xe2x88x92bOe, MeI is Ag, where MeII is Mn, and 0.9 less than b less than 2.1, and 3.8 less than e less than 4.2.
In still another preferred embodiment, the starting mixed metal composition has formula MeIbMeII2xe2x88x92bOe, where MeI is Sb, MeII is Mn, and 1.8 less than b less than 2.2, and 3.8 less than e less than 4.2.
In yet another embodiment, the starting mixed metal composition has formula MeIbMeII2xe2x88x92bOe, MeI is Zn, where MeII is Mn, and 0.9 less than b less than 2.1, and 3.8 less than e less than 4.2.
In another preferred embodiment, the starting mixed-metal composition has the formula MeIbMeII1xe2x88x92bOe, where MeI is Sb, MeII is V, and 0.5 less than b less than 1.5, and 3.8 less than e less than 4.6.
In another preferred embodiment, the starting mixed-metal composition has the formula MeIbMeII1xe2x88x92bOc, where MeI is Sn, MeII is Ti, and 0.2 less than b less than 0.8, and 1.8 less than e less than 2.2.
Another aspect of the invention includes using a starting mixed metal alloy MeIaMeII1xe2x88x92a, or a starting mixed metal compound MeIaMeII1xe2x88x92aXz, where 0.1 less than a less than 0.9, z greater than 0, and X is one of oxygen, boron, carbon, nitrogen, phosphorus, fluorine, chlorine, bromine, or iodine to obtain an electroactive composite material. The starting composition may be a single-phase material, i.e. a solid solution, or a mixed-phase material, i.e., a microphase separated material or a combination of individual phases as is discussed hereinbelow. It is chemically or thermodynamically, partially or internally oxidized so as to produce an oxidized metal compound enriched in MeII within which is a fine metal dispersion enriched in MeI. The term oxidation is understood to mean an increase in the value of z, or an increase in the oxidation state of any one of the metals. The metals MeI are understood to be one or more metals that have a less negative Gibbs free energy for the formation of an alloy or compound with an element X than do the metals that comprise MeII.
When the starting compound is a mixed metal oxide, the metal or metals that have the least negative Gibbs free energy of formation amongst those metal oxides comprising the starting mixed metal oxide form the dispersed metal phase or phases. After the partial oxidation process, the material is useful as a lithium-active component of an electrode in a lithium electrochemical device.
Still another embodiment of the invention uses any of the above methods and materials to prepare a material in which the lithium-active first material and the second material are both oxides, but in which the oxide of the first material is more reduced causing it to have decreased its absolute volume relative to the oxide of the second material. Such a composite material may be considered to be a multiphase oxide rather than a metal-oxide composite. However, it is novel compared to all previously known oxide anodes used in lithium batteries, including the oxides described by Idota et al. and T. Kubota and M. Tanaka, in being a multiphase oxide prepared with a partially reduced lithium-active oxide phase of decreased molar volume, that is subsequently able to alloy with a greater amount of lithium. The composite is characterized by being in a state of internal tension and compression. due to the volume changes experienced during thermochemical treatment.
Yet another aspect of the invention comprises materials in which a starting mixed-metal compound MeIaMeII1xe2x88x92aXz, where MeI, MeII, X, a, and z are as defined above, is mixed with or encompassed within a second material that is a metal or metal alloy, and is then subjected to internal reduction such that said starting compound is reduced, while the second material being initially metallic is not substantially further reduced. In this material the mixed metal compound undergoes volumetric reduction within a matrix or host of the second material which undergoes substantially less volume change.
In each of the above embodiments where a first material and a second material are described, it is understood that each are separate and distinguishable phases, wherein the term xe2x80x9cphasexe2x80x9d is understood to mean a form of condensed matter distinguishable in structure or composition from another. The first and second materials can each also be a mixture of distinguishable phases. After the chemical or thermochemical treatment prescribed the material is useful as a lithium-active component of an electrode in a lithium electrochemical device. In particular, it is useful as the lithium-active anode material in a rechargeable battery using as the cathode a lithium intercalation compound, for example those based on LiCoO2, LiNiO2 or LiMn2O4.
Yet another embodiment of the invention comprises a mixed metal compound MeIaMeII1xe2x88x92aXz, where MeI, MeII, X, a, and z are as defined above, which has been partially reduced so that the value of z is decreased, but without forming a separate metal phase enriched in MeI. The material resulting after partial reduction is a single phase of matter MeIaMeII1xe2x88x92aXz that is more reduced than the starting compound, that is, it has a lower value of z, or the average oxidation state of the metals is lower. The partially reduced material is then able to alloy with lithium to a higher concentration than it can without the partial reduction treatment.
In each of the above embodiments of the invention, the thermochemical treatment prescribed preferably also results in a lithium-active first material and a substantially less active second material that are together in a state of internal stress which is subsequently partly or completely relieved as lithium is inserted into the electrode. For instance, partial reduction causing a decrease in molar volume of the first material can result in a particle of the first material that is in a state of hydrostatic tension, in which case the volume expansion of the metal upon lithiation partly or totally relieves the tensile stress. Another such a stress state is one that is not hydrostatic but has at least one tensile component, in the tensor representation of stress that is well known to those skilled in the art. Another example is a stress state that is predominantly in shear, but in which the strain energy due to shear can nonetheless be reduced by alloying of the metal with lithium. The state of internal stress in the materials of the invention can also be so great as to cause the formation of internal voids, in which case the stress is partly relieved. In this instance, the presence of internal voids also permits a greater amount of lithium to be alloyed with the material than is possible in the absence of the prescribed chemical or thermochemical treatments.
Internal stress and/or internal void space increases the capacity of the material for lithium insertion. For example, compressive and tensile stresses brought about by a volume decrease in the formation of the lithium active first material may be relieved by electrochemical alloying of lithium with the first material since such a process results in a volume increase. Furthermore, void spaces, made in order to accommodate stresses developing in the material, provide additional volume within the composite material which accommodates volume increases from electrochemical alloying of the first material with lithium.
A bulk metal used as an electrode in a conventional lithium ion battery or other such electrochemical system can sometimes be alloyed to higher lithium concentrations than those of known intermetallic compounds in published phase diagrams, some of which are shown in Table 1 (found herein below). This is done by the application of a sufficiently high electrical driving force to insert or alloy lithium into the metal above and beyond that which gives the intermetallic compound in question. The resulting alloy may accept excess lithium to a point, beyond which metallic lithium precipitates and the voltage difference between the alloy and a lithium reference electrode in an electrochemical cell would be zero. For a bulk metal electrode, this kind of material is not substantially different from having a lithium metal electrode, since lithium metal is present as a distinct phase in the metal electrode. The limitations of lithium metal electrodes, such as poor safety upon exposure to water, are present in such a material, as are the limitations associated with volume expansion of the alloy.
In contrast, in the materials of the invention, overlithiation of the metal to compositions richer in lithium than those shown in published phase diagrams is possible without suffering these negative consequences. This is possible, firstly, because the first material that is alloyed with lithium is substantially protected from exposure to a reactive environment by being partially or completely surrounded by a less reactive second material, and secondly, because the lithium-active first material is prepared in a state of decreased molar volume prior to assembly into an electrochemical device. The materials of the invention are therefore distinguished from previous lithium electrodes made of the same metals, but not under conditions of practical reduction or oxidation, in having better retention of charge capacity upon electrochemical cycling over the same voltage range relative to lithium metal. In particular, the materials of the invention have a high weight or volumetric charge capacity over a voltage range relative to lithium metal as the counter electrode that is less than 2 volts. They are further distinguished by having a high weight or volumetric charge capacity over many charging and discharging cycles. These results are unexpected to those skilled in the art.
The materials of the invention are readily produced in the form of a film or sheet by deposition processes such as sputtering and evaporation, using one or more targets or sources, or by chemical vapor deposition using gas phase reactants, or by electrochemical deposition, or by slurry-based processes such as spin-casting, dip-coating, and tape-casting, or by mechanical deformation processes such as rolling, swaging, and extrusion. Thus the invention also comprises the materials of the invention prepared in the form of a thin film or a sheet, and devices made from or containing such forms of the materials.